The present invention is directed to bicycle transmissions and, more particularly, to a bicycle hub transmission that includes a shift key that is axially immovable relative to a clutch member used to shift gears in the transmission.
Three-speed internal shifter hubs are commonly mounted to the rear part of the bicycle frame. Such internal shifter hubs typically comprise a hub axle fixed to the rear part of the bicycle frame, a driver capable of rotating about the hub axle, a hub shell fitted over the driver, a power-transmitting mechanism having three power transmission paths (upshift, downshift, and direct) and comprising a planetary gear mechanism and a plurality of one-way clutches, and a switching mechanism for switching among the power transmission paths of the power-transmitting mechanism.
The driver is provided with a hub cog, and motive power is transmitted to the driver from the chainwheel crank through a chain. The hub shell, which is a cylindrical member having a space in its interior, is provided along the external peripheral portion thereof with hub flanges for attaching the spokes for the rear wheel. The planetary gear mechanism comprises a sun gear formed integrally with the hub axle, a cage rotatably supported around the hub axle, planetary gears rotatably mounted in the cage and meshing with the sun gear, and a ring gear that meshes with the planetary gears. One-way clutches are disposed between the driver and the ring gear, between the ring gear and the hub shell, and between the cage and the hub shell in order to form three power transmission paths. The one-way clutch disposed between the ring gear and the hub shell can be alternated between a linked state and a disengaged state by the switching mechanism, and the other one-way clutches are used solely to transmit one-way rotation (rotation in the direction of travel of the bicycle) from the cage to the hub shell or from the driver to the ring gear.
The switching mechanism, which is rotatably supported and axially movable on the axle, comprises a cylindrical clutch member for selecting one of the three power transmission paths. The central portion of the hub axle is provided with a guide hole extending in the axial direction from one end to the center, and the center of the hub axle at the end portion of the guide hole is provided with a slot extending through the axle in the radial direction. An axially movable shift rod is disposed in the guide hole of the hub axle, and a shift key is mounted in the axle slot for moving the clutch element in response to axial movement of the shift rod. The clutch member can be moved into one of the following three shift positions: an upshift position; a direct-link position; and a downshift position. The clutch member is moved into the various shift positions by pushing the shift rod with a bell crank linked by a shifter cable to a shift lever, twist grip, or other type of shift control device. A first biasing member biases the clutch member toward the upshift position, opposite the pushing direction of the shift rod.
When the clutch member is in the upshift position, the driver and the cage of the planetary gear mechanism are linked to each other by the clutch member. In this state, the cage rotates at the same rotational velocity as the driver, the ring gear is upshifted through the planetary gears, and the motive power of the driver is thereby upshifted and transmitted to the hub shell through through the one-way clutch disposed between the ring gear and the hub shell.
Moving the shift control by one click stop pivots the bell crank which, in turn, pushes the shift rod against the biasing force of the first biasing member, and the shift key moves the clutch member into the direct-link position. When this occurs, the connection between the driver and the cage is released, and the motive power of the driver is directly transmitted to the ring gear through the one-way clutch disposed between the driver and the ring gear. The motive power of the driver then is transmitted directly from the ring gear to the hub shell through the one-way clutch disposed between the ring gear and the hub shell.
Moving the shift control by yet another click stop further pivots the bell crank which, in turn, pushes the shift rod, and the shift key moves the clutch member into the downshift position. When this occurs, the one-way clutch disposed between the ring gear and the hub shell is disengaged by the clutch member, and the motive power of the driver is downshifted by being transmitted from the driver to the ring gear through the one-way clutch disposed between the driver and the ring gear. The motive power of the driver is downshifted through the planetary gears to the cage, and the motive power then is transmitted from the cage to the hub shell through the one-way clutch disposed between the cage and the hub shell.
The shift key is housed inside the clutch member and is allowed to move in the axial direction integrally with the clutch member. The first biasing member, which biases the clutch member toward the upshift position, also biases the clutch member toward the shift key, and a second biasing member is provided for biasing the shift key toward the clutch member so that the shift key follows the clutch member when the clutch member is moved by the shift rod. Thus, the first biasing member and the second biasing member bias the shift key and the clutch member in opposite directions toward each other. When the shift control is moved back to obtain the direct drive or upshifted speeds, the clutch member and the shift key move in the direction toward the cage as a result of the biasing force of the first biasing member.
Sometimes the clutch member disengages from the cage against the biasing force of the first biasing member when the driver rotates in a state of slight meshing between the clutch member and the cage. In such cases, the shift key sometimes disengages from the clutch member without following it when a second biasing member with a light biasing force is used. Disengagement of the shift key from the clutch member creates the risk that the shift key will be tilted and rendered incapable of pushing the clutch member any longer.
The biasing force of the second biasing member can be increased to ensure that the shift key remains with the clutch member and thereby prevent this phenomenon. However, increasing the biasing force of the second biasing member makes it necessary to further increase the biasing force of the first biasing member so that the second biasing member will not override the operation of the first biasing member and inhibit the clutch member from moving toward the cage when the shift rod is released. Unfortunately, increasing the biasing force of the first biasing member also increases the force needed to perform shifting operations. It is therefore preferable for the biasing force of the second biasing member to be minimal.